Early work relating to network-based Wireless Location Systems (WLS) is described in U.S. Pat. No. 4,728,959; “Direction Finding Localization System” (issued Mar. 1, 1998) which discloses a system for locating cellular telephones using angle of arrival (AOA) techniques and U.S. Pat. No. 5,327,144, (Issued Jul. 5, 1994) “Cellular Telephone Location System,” which discloses a system for locating cellular telephones using time difference of arrival (TDOA) techniques. Further enhancements of the system disclosed in the '144 patent are disclosed in U.S. Pat. No. 5,608,410, (Issued Mar. 4, 1997), “System for Locating a Source of Bursty Transmissions”. Location estimation techniques for wide-band wireless communications systems were further developed in U.S. Pat. No. 6,047,192 (Issued Apr. 4, 200), “Robust, Efficient Localization System”.
All of these patents are assigned to TruePosition, Inc., the assignee of the present invention. TruePosition has continued to develop significant enhancements to the original inventive concepts. First commercially deployed in 1998, overlay network-based wireless location systems have been widely deployed in support of location-based services including emergency services location. The use of multiple location calculations for a single location estimate was previously explored in U.S. Pat. No. 6,097,336; “Method for improving the accuracy of a wireless location system”, by Stilp, Issued Aug. 1, 2000; U.S. Pat. No. 6,334,059; “Modified transmission method for improving accuracy for e-911 calls” by Stilp et al. Issued Dec. 25, 2001; U.S. Pat. No. 6,603,428; “Multiple pass location processing”; by Stilp, issued Aug. 5, 2003; and U.S. Pat. No. 6,873,290; “Multiple pass location processor”, by Anderson et al., issued Mar. 29, 2005.
A U-TDOA location system (and other location systems) location performance is normally expressed as one or more circular error probabilities. The United States Federal Communications Commission (FCC) as part of the 2001 Enhanced 9-1-1 Phase II mandate requires that network-based systems, such as U-TDOA, be deployed to yield a precision that generates a one-hundred meter (100 m or 328.1 feet) accuracy for a yield of 67% of emergency services callers and a three-hundred meter (300 m or 984.25 feet) accuracy for a yield of 95% of emergency services callers. In 2011, the FCC set a new single location accuracy requirement, to be implemented in 2019 for any and all E911 location technologies, to 50 meters for 67% emergency services callers and 150 meters accuracy for 95% of emergency services callers. This legal requirement makes location accuracy (and yield) of paramount importance for wireless location systems. As realized and noted in prior art, the ability to routinely, reliably, and rapidly locate cellular wireless communications devices has the potential to provide significant public benefit in public safety and convenience and in commercial productivity.
The inventive techniques and concepts described herein apply to time and frequency division multiplexed (TDMA/FDMA) radio communications systems including the widely used GSM; the OFDM based LTE, LTE-Advanced wireless systems and WiMAX systems, as well as with code-division radio communications systems such as CDMA (IS-95, IS-2000) and Universal Mobile Telecommunications System (UTMS), the latter of which is also known as W-CDMA.
The 3GPP-defined E-UTRAN (Evolved Universal Mobile Telecommunications System Universal Mobile Telecommunications System Terrestrial Radio Access Network), also known as Long Term Evolution or simply as LTE wireless communications network model and terminology used in this specification is an exemplary but not exclusive environment in which the present invention may be used.